The present invention relates to an electrically responsive light controlling device, and more particularly to a light controlling device having an excellent controllability of light transmittance.
In recent years, a demand for controlling the transmittance of window glasses is increasing in various fields using a glass. Among others, in the fields of building and vehicles, it is strongly demanded from the viewpoint of energy saving to impart a light controlling ability to a glass, because entering of solar radiation energy into a room through windowpanes greatly affects the load of air conditioning in room. Also, there is growing interest in a glass having a light controllability from the viewpoint of utilizing it as a so-called display device by locally and selectively changing the transmittance and/or reflectance of the glass.
With respect to such materials and devices having a light transmission controllability, attention has been paid for the last few decades to an electrochromic device (EC device) using an electrochemically color developing material such as tungusten oxide, molybdenum oxide or nickel oxide. The studies thereof have been earnestly continued in various fields, and the technique has advanced to such a degree as being put to practical use on small glass products, such as glasses and automobile mirrors. However, these products driven by an electric current leads to an inevitable voltage drop and an increase in the surface area results in a marked decrease in speed of response. In addition, a side reaction, accelerated by injection and rejection of a large amount of electricity, causes degradation of the constituting materials. For such reasons, it has become clear that it is very difficult to realize practical EC devices having a large area by merely utilizing the fabrication technique for EC devices having a small area.
As devices to be substituted for such EC devices of electric current actuation type, attention is paid to light controlling devices of voltage actuation type. The representative example is a liquid crystal device using encapsulated liquid crystal as disclosed in Japanese Patent Publication Kokai No. 58-501631. This device merely controls the parallel transmission by change in a degree of diffusion of transmitted light based on control of orientation of liquid crystal molecules and, therefore, it is unsatisfactory for controlling energy transmission which is demanded for light controlling devices.
For this reason, attention is being paid to a light controlling device wherein anisotropic particles having a strong light absorptivity are suspended in a liquid dielectric medium and the light transmittance of the suspension is controlled by applying an electric field to the suspension (the device of this type being hereinafter referred to as "DPS (Dipole Particle Suspension) device") in order to provide a light controlling device of voltage actuation type which is considered that large area application would be relatively easy and which can also control energy transmission in a wide range. The DPS device called light valve is known from the mid-1960s, and proposals concerning the DPS device have been made, for example, in U.S. Pat. No. 3,257,903 and Japanese Patent Publication Kokai No. 51-69038. Proposed anisotropic particles suitable for the DPS device include a halide such as a perhalide of an alkaloid acid salt as represented by herapathite, a polarizing metal halide or other halides; fine particles of an organic compound such as nafoxidine hydrochloride or guanine; fine particles of an inorganic compound such as basic lead carbonate, bismuth oxychloride, lead hydrogenarsenate, lead hydrogenphosphate, graphite, mica or garnet; and mica or glass flakes covered with a metal or metal oxide such as aluminum, chromium, gold, palladium, silver, tantalum, titanium, tin oxide, titanium oxide or vanadium pentaoxide. Among others, attempts to put the DPS device into practical use using herapathite particles have been earnestly made. On the other hand, in the case of DPS devices using herapathite particles, an electrically induced optical change in near-infrared region is negligible and the particles themselves are poor in UV resistance and, therefore, application to control of solar energy transmission, namely to a light controlling glass, has been considered very difficult.
In view of such circumstances, the present inventors have already proposed some improved anisotropic particles, e.g. particles covered with a metal or an organometallic compound (Japanese Patent Publication Kokai No. 64-38732), particles covered with a tungsten bronze film (Japanese Patent Publication Kokai No. 64-57242) and particles covered with a compound of the formula: Ti.sub.n O.sub.2n-1 (Japanese Patent Publication No. 1-126629). These particles have a high light absorptivity and a good resistance to ultraviolet rays. However, problems are encountered in putting a DPS device using these particles into practical use. That is to say, the particles covered with a metal have the problems that it is difficult to form an insulation film having a good adhesiveness on the metal film, and that the dispersibility of the particles is not satisfactory. The particles covered with tungsten bronze have the drawbacks that they are dissolved in even a slight amount of water, and that positive holes produced by absorbing ultraviolet rays oxidatively decompose surrounding organic materials. Also, in the case of the particles covered with a compound of the formula: Ti.sub.n O.sub.2n-1, realization of properties sufficient for light controlling glass requires one to increase the absorption coefficient and/or to increase the covering thickness because the absorption characteristics are determined by both the absorption coefficient and the thickness of the covering material. However, any of the particles capable of practically functioning as a DPS device have upper limits thereof and, therefore, the controllable range for solar energy transmission is at most 16% even with the use of the best particles.
In general, as for the light controlling range desired in practical use of a device having a light controlling ability like a DPS device, it is considered that the range must exceed the difference of transmittance between a transparent window glass and a heat absorbing window glass intended to absorb solar energy. Accordingly, it may be said that the required control range is at least 20% in terms of variation (.DELTA.T) of solar energy transmittance. This light control range has not been attained by any known anisotropic particles.
It is a primary object of the present invention to provide a light controlling device which can control the solar energy transmittance with the variation .DELTA.T of at least 20%.
A further object of the present invention is to provide a light controlling device which has excellent controllability of light transmittance and solar energy transmittance and which is suitable for use as a light controlling glass, a display device of light transmitting or light reflecting type, and for other various purposes.
These and other objects of the present invention will become apparent from the description hereinafter.